A joint research team from Houston’s Rice University and Baylor College of Medicine says it is one step closer to creating implantable tissues with functioning capillaries—a crucial step in the direction of 3D printing transplantable tissues and organs.

The research, recently published in the journal Biomaterials Science under the title “Tubulogenesis of co-cultured human iPS-derived endothelial cells and human mesenchymal stem cells in fibrin and gelatin methacrylate gels,” was led by Rice University bioengineer Jordan Miller and Baylor biophysicist Mary Dickinson.

Together, and with their dedicated teams, Miller and Dickinson have demonstrated how to combine human endothelial cells with mesenchymal stem cells to trigger tubulogenesis, a process that enables blood-transporting capillaries to form.

"Our work has important therapeutic implications because we demonstrate utilization of human cells and the ability to live-monitor their tubulogenesis potential as they form primitive vessel networks," explained Gisele Calderon, a graduate student in Miller's Physiologic Systems Engineering and Advanced Materials Laboratory and lead author of the study.

"We've confirmed that these cells have the capacity to form capillary-like structures, both in a natural material called fibrin and in a semisynthetic material called gelatin methacrylate, or GelMA,” she continued. “The GelMA finding is particularly interesting because it is something we can readily 3D print for future tissue-engineering applications.”

3D printing and bioprinting technologies have offered a unique opportunity for researchers to explore tissue engineering and regenerative medicine. Though many scientists have succeeded in printing or building stem cells into particular structures, one big hurdle has been the ability to grow tissues with vasculature (capillaries and blood vessels that are crucial for giving tissues nutrients and, consequently, life).

As the researchers explain, without vascularization any tissue that is more than a few millimeters in thickness will not survive. That’s why the research being done at Rice University and the Baylor College of Medicine is so exciting.

“Ultimately, we'd like to 3D print with living cells, a process known as 3D bioprinting, to create fully vascularized tissues for therapeutic applications,” said Jordan Miller, an assistant professor of bioengineering who has studied vascularization and tissue engineering for more than a decade.

"To get there, we have to better understand the mechanical and physiological aspects of new blood vessel formation and the ways that bioprinting impacts those processes. We are using 3D bioprinting to build tissues with large vessels that we can connect to pumps, and are integrating that strategy with these [induced pluripotent stem and endothelial cells] iPS-ECs to help us form the smallest capillaries to better nourish the new tissue.”

Made from networks of endothelial cells, capillaries are the small blood vessels that supply the body with oxygen and nutrients—some are so narrow and small that blood cells are carried through them in a single-file fashion. Tubulogenesis, the process that the researchers have managed to initiate, is essentially the first stage of producing capillaries.

In short, tubologenesis is the process wherein endothelial cells form vacuoles (tiny, empty chambers) and subsequently connect to nearby cells which allow the vacuoles to link together to form endothelial-lined tubes which ultimately become capillaries. Being able to generate this process in a lab is a significant achievement.

“We expect our findings will benefit biological studies of vasculogenesis and will have applications in tissue engineering to prevascularize tissue constructs that are fabricated with advanced photo-patterning and three-dimensional printing," commented Mary Dickinson, the Kyle and Josephine Morrow Chair in Molecular Physiology and Biophysics at the Baylor College of Medicine and an adjunct professor of bioengineering at Rice University.

The research conducted by the joint team set out to determine whether commercially available endothelial cells (from iPSCs) could be used to trigger tubulogenesis. The experiments conducted by the team consisted of using the iPSCs as well as human mesenchymal stem cells in two types of semisolid gels: fibrin, a natural material known for inducing tubulogenesis; and GelMA, a mixture of “denatured collagen that was chemically modified with methacrylates to allow rapid photopolymerization.”

After many experiments and months of research, the team finally succeeded in establishing a workflow for producing “robus tubulogenesis” in GelMA. The process necessitated the addition of mesenchymal stem cells, which help to stabilize the formation of tubules.

According to Miller, his team’s research could lead to advances in clinical applications such as drug testing. He explains: “You could foresee using these three-dimensional, printed tissues to provide a more accurate representation of how our bodies will respond to a drug. Preclinical human testing of new drugs today is done with flat two-dimensional human tissue cultures. But it is well-known that cells often behave differently in three-dimensional tissues than they do in two-dimensional cultures.”

“There’s hope that testing drugs in more realistic three-dimensional cultures will lower overall drug development costs," Miller adds. "And the potential to build tissue constructs made from a particular patient represents the ultimate test bed for personalized medicine. We could screen dozens of potential drug cocktails on this type of generated tissue sample to identify candidates that will work best for that patient."